Arvinder Sandhu

Soft X-ray-Driven Femtosecond Molecular Dynamics

The direct observation of molecular dynamics initiated by x-rays has been hindered to date by the lack of bright femtosecond sources of short-wavelength light. We used soft x-ray beams generated by high-harmonic upconversion of a femtosecond laser to photoionize a nitrogen molecule, creating highly excited molecular cations. A strong infrared pulse was then used to probe the ultrafast electronic and nuclear dynamics as the molecule exploded. We found that substantial fragmentation occurs through an electron-shakeup process, in which a second electron is simultaneously excited during the soft x-ray photoionization process. During fragmentation, the molecular potential seen by the electron changes rapidly from nearly spherically symmetric to a two-center molecular potential. Our approach can capture in real time and with angstrom resolution the influence of ionizing radiation on a range of molecular systems, probing dynamics that are inaccessible with the use of other techniques.

Response of graphene to femtosecond high-intensity laser irradiation

We study the response of graphene to high-intensity, 50-femtosecond laser pulse excitation. We establish that graphene has a high (∼3 × 1012 Wcm−2) single-shot damage threshold. Above this threshold, a single laser pulse cleanly ablates graphene, leaving microscopically defined edges. Below this threshold, we observe laser-induced defect formation leading to degradation of the lattice over multiple exposures. We identify the lattice modification processes through in-situ Raman microscopy. The effective lifetime of chemical vapor deposition grown graphene under femtosecond near-infrared irradiation and its dependence on laser intensity is determined. These results also define the limits of non-linear applications of graphene in femtosecond high-intensity regime.

Observing the creation of electronic feshbach resonances in soft x-ray-induced O2 dissociation.

When an atom or molecule is ionized by an x-ray, highly excited states can be created that then decay, or autoionize, by ejecting a second electron from the ion. We found that autoionization after soft x-ray photoionization of molecular oxygen follows a complex multistep process. By interrupting the autoionization process with a short laser pulse, we showed that autoionization cannot occur until the internuclear separation of the fragments is greater than approximately 30 angstroms. As the ion and excited neutral atom separated, we directly observed the transformation of electronically bound states of the molecular ion into Feshbach resonances of the neutral oxygen atom that are characterized by both positive and negative binding energies. States with negative binding energies have not previously been predicted or observed in neutral atoms.

IR-assisted ionization of helium by attosecond extreme ultraviolet radiation

Attosecond science has opened up the possibility of manipulating electrons on their fundamental timescales. Here, we use both theory and experi- ment to investigate ionization dynamics in helium on the attosecond timescale by simultaneously irradiating the atom with a soft x-ray attosecond pulse train (APT) and an ultrafast laser pulse. Because the APT has resolution in both energy and time, we observe processes that could not be observed without resolu- tion in both domains simultaneously. We show that resonant absorption is impor- tant in the excitation of helium and that small changes in energies of harmonics that comprise the APT can result in large changes in the ionization process. With the help of theory, ionization pathways for the infrared-assisted excitation and ionization of helium by extreme ultraviolet (XUV) attosecond pulses have been identified and simple model interpretations have been developed that should be of general applicability to more complex systems (Zewail A 2000 J. Phys. Chem. A 104 5660-94).

Time-resolved momentum imaging system for molecular dynamics studies using a tabletop ultrafast extreme-ultraviolet light source

We describe a momentum imaging setup for direct time-resolved studies of ionization-induced molecular dynamics. This system uses a tabletop ultrafast extreme-ultraviolet (EUV) light source based on high harmonic upconversion of a femtosecond laser. The high photon energy (around 42 eV) allows access to inner-valence states of a variety of small molecules via single photon excitation, while the sub--10-fs pulse duration makes it possible to follow the resulting dynamics in real time. To obtain a complete picture of molecular dynamics following EUV induced photofragmentation, we apply the versatile cold target recoil ion momentum spectroscopy reaction microscope technique, which makes use of coincident three-dimensional momentum imaging of fragments resulting from photoexcitation. This system is capable of pump-probe spectroscopy by using a combination of EUV and IR laser pulses with either beam as a pump or probe pulse. We report several experiments performed using this system.

Long-term carrier-envelope phase stability from a grating-based, chirped pulse amplifier

We demonstrate a carrier-envelope phase (CEP) stabilized, chirped pulse laser amplifier that exhibits greatly improved intrinsic long-term CEP stability compared with that of other amplifiers. This system employs a grating-based stretcher and compressor and a cryogenically cooled laser amplifier. Single-shot carrier envelope phase noise measurements are also presented that avoid underestimation of this parameter caused by fringe averaging and represent a rigorously accurate upper limit on CEP noise.

Enhanced hard x-ray emission from microdroplet preplasma

We perform a comparative study of hard x-ray emission from femtosecond laser plasmas in 15μm methanol microdroplets and Perspex target. The hard x-ray yield from droplet plasmas is ≃68 times more than that obtained from solid plasmas at 2×1015Wcm−2. A 10ns prepulse at about 5% of the main pulse appears to be essential for hard x-ray generation from droplets. Hot electron temperature of 36keV is measured from the droplets at 8×1014Wcm−2, whereas a three times higher intensity is needed to obtain similar hot electron temperatures from Perspex plasmas. Particle-in-cell simulations with very long scale-length density profiles support experimental observations.

A low-cost spatial light modulator for use in undergraduate and graduate optics labs

Spatial light modulators (SLMs) are a versatile tool for teaching optics, but the cost associated with an SLM setup prevents its adoption in many undergraduate and graduate optics labs. We describe a simple method for creating a low-cost SLM by extracting components from a commercial LCD projector. We demonstrate the pedagogical applications of this SLM design by providing examples of its use in teaching diffraction and interference phenomena. We also discuss an SLM’s potential as a research tool in graduate labs. In particular, we demonstrate its use in holography and in the generation of optical vortices.

Time resolved evolution of structural, electrical, and thermal properties of copper irradiated by an intense ultrashort laser pulse

The dynamical properties of copper metal are obtained on a picosecond time scale using 100fs laser pulse at 1015Wcm−2—an intensity regime relevant to femtosecond micromachining. The dissipation mechanisms and scaling laws spanning a wide temperature range are obtained from femtosecond pump–probe reflectivity. We observe obliteration of the crystalline structure in copper within 400fs due to lattice disorder caused by the intense laser pulse. The electrical resistivity is obtained by studying the probe reflectivity evolution from 0to30ps. The “resistivity saturation” effect in an unexplored regime intermediate to hot plasma and cold solid is studied in detail. The temperature evolution and thermal conductivity values are also obtained.

Ultrafast dynamics of neutral superexcited oxygen: a direct measurement of the competition between autoionization and predissociation.

Using ultrafast extreme ultraviolet pulses, we performed a direct measurement of the relaxation dynamics of neutral superexcited states corresponding to the nlσ(g)(c(4)Σ(u)(-)) Rydberg series of O(2). An extreme ultraviolet attosecond pulse train was used to create a temporally localized Rydberg wave packet and the ensuing electronic and nuclear dynamics were probed using a time delayed femtosecond near-infrared pulse. We investigated the competing predissociation and autoionization mechanisms in superexcited oxygen molecules and found that autoionization is dominant for the low n Rydberg states. We measured an autoionization lifetime of 92±6 fs and 180±10 fs for the (5s,4d)σ(g) and (6s,5d)σ(g) Rydberg state groups, respectively. We also determine that the disputed neutral dissociation lifetime for the ν=0 vibrational level of the Rydberg series is 1100±100 fs.